1. Field of the Invention
The present invention relates to a method for forming contact holes. More particularly, the present invention relates to a method for forming contact holes by utilizing two or more exposures and two or more etchings.
2. Description of the Prior Art
With the advancing technology of the semiconductor industry, integrated circuits are being developed to increase the current computing and storage capability. As predicted by Moore's law, the number of transistors doubles every 18 months. The process of semiconductor evolves from 0.18 μm of 1999, 0.13 μm of 2001, 90 nm (0.09 μm) of 2003 to 65 nm (0.065 μm) of 2005 and is approaching 45 nm. Therefore, the density of semiconductor elements on a wafer is increasing with the technology advancement of the semiconductor industry and miniaturization of microelectronic elements and makes the intervals between elements shorter and shorter, which increases the difficulty of the etching process for contact holes.
In prior art methods for producing contact holes, the photo resist layer serves as an etching mask for etching the underlying dielectric layer. For the 45 nm process, the pitch (the distance of centers of two neighboring contact holes) for contact holes must be lower than 155 nm and the “after development inspect critical dimension” (ADICD) must be around 70-80 nm. For the current lithographic tools, it is impossible to create contact holes with pitch lower than 155 nm in one exposure. The current solution is that the desired contact holes are patterned by two exposures with two photo masks on a photo resist layer, and then followed by one etching step.
However, the problem is that when the pitch of contact holes is lower than 140 nm, the above-mentioned two-exposures-then-one-etching method fails because it is beyond the limitation of the current lithographic tools during the second exposure, and consequently contact holes lower than 140 nm pitch cannot be produced.
Accordingly, it is an important issue to achieve contact holes of pitch lower than 140 nm in this regard.